/etc/ppp/options:
-detach
noauth
noipdefault
usepeerdns
asyncmap 0
modem
crtscts
lock
proxyarp
ms-dns 192.168.100.1
ipcp-accept-local
ipcp-accept-remote
deflate 0

/etc/ppp/options.ttyS0:
192.168.100.1:192.168.100.100
netmask 255.255.255.0

/etc/mgetty+sendfax/login.config:
/AutoPPP/ -     a_ppp   /usr/sbin/pppd auth -chap +pap login debug
*       -       -       /bin/login @

/etc/mgetty+sendfax/mgetty.config:
port ttyS0
    debug 4
    speed 115200
    modem-type data
    init-chat "" AT&F1&B1&A3E0Q0V1&C1&D2&P1S0=0 OK ATS0=0 OK AT#CID=1 OK